The field of the invention relates generally to electrical contactors, and more particularly, a freewheel circuit for a contactor.
A contactor, or relay, is an electromagnetic device operable to selectively open and close one or more electrical contacts in response to a voltage applied to a coil in the contactor. FIGS. 1 and 2 are circuit diagrams of known contactor circuits 1 and 5, respectively.
In contactor circuit 1, in a quiescent state, a transistor 2 (“TR1”) is turned off and a voltage at its collector is V1. When a positive control voltage V2 of a predetermined magnitude is applied to a base of transistor 2, the resultant current flow through a relay coil 3 from V1 to ground establishes an electromagnetic field in relay coil 3 that causes a contact 4 to close. At this point, most of the V1 voltage will be developed across relay coil 3 and the voltage on the collector of transistor 2 will be minimal. When the control voltage falls below a certain level, transistor 2 turns off and interrupts current flow through relay coil 3, causing collapse of the electromagnetic field and immediate opening of contact 4. However, the energy stored in relay coil 3 cannot be dissipated immediately, setting up a back EMF that results in a voltage substantially greater than V1 appearing on the collector of transistor 2. Depending on the rating of transistor 2, this voltage could result in the breakdown and/or failure of transistor 2.
This issue is overcome by the arrangement of contactor circuit 5, where a diode 6 has been connected in inverse parallel across relay coil 3. Under normal conditions, diode 6 is non-conducting. However, when transistor 2 is turned off, the voltage rise at the collector of transistor 2 will cause diode 6 to conduct and clamp the collector voltage to about 0.7 volts (V) above V1, preventing damage to transistor 2. However, current flow will be maintained in the current loop formed by relay coil 3 and diode 6, and this current flow will reduce relatively slowly over an indefinite period until such time as the energy in relay coil 3 has been sufficiently dissipated to open contact 4. This relatively slow dissipation results in a gradual opening of contact 4 instead of a sudden opening, which increases the risk of sustained arcing across contact 4 and resultant damage to contact 4. The issues of slow energy dissipation within contactor circuit 5 may be mitigated to some extent by using active components rather than diode 6 alone.
The current required to energize a contactor coil (e.g., relay coil 3) sufficiently to close the contacts (referred to as a closing current) is substantially greater than the current required to keep the contacts in the closed state (referred to as a holding current). Once the coil current falls below the holding current level, the contacts will open automatically. If energy stored in the coil is harnessed to maintain the contacts in the closed state for a certain period of time, it is possible to remove the closing current temporarily, restoring it at regular intervals. In effect, the closing current may be switched on and off at regular intervals, so long as the contacts are maintained in the closed state during the off periods. This reduces the mean external current required to maintain the contacts in the closed state.
FIG. 3 is a circuit diagram of a known freewheel circuit 10 that includes a first coil 12 (“L1”) and a second coil 14 (“L2”) in series with a first transistor 16 (“Q1”). A first voltage 18 (“V1”) provides the closing current for the contactor. A second voltage 20 (“V2”) provides a control voltage that is initially in the form of a steady state voltage operable to turn on first transistor 16. When first transistor 16 is turned on, a closing current flows in a first current loop 22 (“I1”) through the series chain of first coil 12, second coil 14, first transistor 16, and a first resistor 24 (“R4”). First coil 12, a Darlington transistor pair 30 (“Q2”), and a first diode 32 (“D1”) form a second current loop 34 (“I2”). Second coil 14, a second diode 40 (“D2”), and a first Zener diode 42 (“ZD1”) form a third current loop 44 (“I3”).
When current ceases to flow in third current loop 44, energy stored in first and second coils 12 and 14 will cause the voltage at the drain of first transistor 16 to rise substantially above V+. If left uninterrupted, this voltage rise may result in damage to first transistor 16. However, the voltage rise causes a pulse of current to flow through first diode 32, a capacitor 50, and emitters of Darlington pair 30 to V+, turning on Darlington pair 30. This results in a voltage drop across Darlington pair 30 of approximately 1V and starts circulation of current within second current loop 34 to maintain contactor contacts (not shown in FIG. 3) in the closed state and prevent escalation of the voltage on first transistor 16. As capacitor 50 acquires charge, the current flow to Darlington pair 30 from capacitor 50 will decrease. However, when the voltage across capacitor 50 exceeds a breakover voltage of a second Zener diode 52 (“ZD2”), current will be supplied to Darlington pair 30 through second Zener diode 52 to keep Darlington pair 30 on. At this stage, the voltage across Darlington pair 30 will rise to a level slightly higher than the breakover voltage of second Zener diode 52, thus clamping the voltage across Darlington pair 30 to this level.
The voltage rise across second coil 14 gives rise to a current in third current loop 44, and this voltage will be clamped by first Zener diode 42 and second diode 40 while the energy in second coil 14 is dissipated. When this current flows, first diode 32 and Darlington pair 30 are forward biased. When first transistor 16 turns on again, second coil 14 acts as a snubber coil to mitigate any risks of reverse break-over of first diode 32 and Darlington pair 30.
Second Zener diode 52 and a third diode 60 (“D3”) clamp a voltage across Darlington pair 30 to facilitate preventing Darlington pair 30 from being stressed by relatively high voltages. For the clamping to work, however, capacitor 50 should be discharged to ensure it can pass a current pulse to Darlington pair 30 immediately after first transistor 16 is turned off. This is achieved by using a second resistor 62 (“R1”) that provides a discharge path for capacitor 50. However, this results in power dissipation in third diode 60, second Zener diode 52, and second resistor 62, and also diverts current that could be flowing through first coil 12 to a parallel circuit, reducing the overall efficiency of circuit 10.
Further, the current in second current loop 34 may be relatively high (e.g., greater than 3 A) such that the power dissipation across Darlington pair 30 is relatively high (e.g., greater than 3 Watts (W)), requiring Darlington pair 30 to have a relatively high power rating. Moreover, when current is flowing through second current loop 34, the total power dissipation in Darlington pair 30 and first diode 32 may be relatively high (e.g., 5 W for a current of 3 A), reducing the overall efficiency of circuit 10.